CH657355A5 - PROCESS FOR THE MANUFACTURE OF HOLLOW GLASS BEADS FILLED WITH GAS. - Google Patents

Description

The present invention relates to a method of manufacturing hollow glass beads filled with gas.

A particular use of hollow glass beads filled with gas is in the field of explosive compositions of the emulsion type where they are used as a filler, for example as described in British patent applications 1,593,163 (Atlas Powder Company) and 2,055,358 (EI Du Pont de Nemours and Company), corresponding respectively to French patents 2,370,015 and 2,463,110.

For example, Atlas Powder's application describes an explosive primer-sensitive composition consisting of an amount of 3 to 10% by weight of carbonaceous fuel comprising an emulsifier, a discontinuous aqueous phase of oxidizing solution containing from 10 to 25% by weight of and from 35 to 86% by weight of oxidizing inorganic salts composed mainly of ammonium nitrate and materials comprising closed cell voids sufficient to obtain a density of the explosive emulsion composition of between 1.1 and 1.3 g / cm3. It is established therein that the materials comprising closed cell voids preferred for this use are glass microbubbles having a size between 10 and 175 µm and an apparent density between 0.1 and 0.4 g / cm3, which can constitute from 1 to 10% by weight of the explosive mixture. It is implied that these glass beads may be present in amounts exceeding 40% of the total volume of the mixture. The incorporation of such glass beads in the explosive emulsion allows the formulation of a composition sensitive to a conventional electric primer No. 6 and not detonated by fire, by bullet, by impact, by friction or by static electricity.

Hollow glass beads having a size and density within these ranges can easily be formed by spraying an aqueous solution or suspension of glass forming products, so that droplets of the solution pass through a heating zone which is at a temperature capable of causing the formation of glass. Cell formation is then caused by the evaporation of water inside each droplet after an outer skin is formed by heating. If desired, a substance such as urea, which gives off gas at the heating temperature, can be added to the mixture. Specific examples of such a process are described in British patent application 1,568,817 (Sovitec S.A.).

When glass beads are formed in this way, they have certain properties which could be improved for the intended purpose.

In particular, the pearls, being formed largely from soluble glass-forming materials, are liable to be attacked by hydrolysis. They will therefore age when incorporated into a water-in-oil emulsion, thereby limiting the shelf life of the explosive.

In addition, the gas pressure inside the hollow glass beads at the time of cooling after manufacture is 0.3 to 0.4.105 Pa and this low pressure tends to act as a brake on the detonation rate of a explosive in which they are incorporated.

The object of the present invention is to provide a method of manufacturing hollow glass beads filled with gases which have improved properties for their incorporation in emulsion type explosives.

The present invention provides a method of manufacturing hollow glass beads filled with gas, characterized in that it comprises a step of manufacturing hollow glass beads having an alkali content of at least 20%, calculated as a percentage by weight of oxide in the pearls, a stage of treatment of the pearls by means of an acid medium to reduce the alkali content of the pearls to a value which is not more than 15%, and a stage during which it is made to diffuse gas inside the beads.

Glass beads produced according to the invention have highly desirable properties for their incorporation into emulsion type explosives. The use of beads filled with gas allows the formulation of an emulsion which can be detonated uniformly and reliably. Because the alkali content of the beads is not more than 15% by weight, they are resistant to hydrolytic attack so that this uniformity and reliability is maintained for a suitable storage time.

It is established in the British patent application 2,055,358 cited above that it is not desirable to incorporate more than 50% by volume of gas bubbles or voids in the emulsion, since it can result in a weak detonating power. This reduction in the detonating power is avoided by diffusing gas inside the glass beads, so that their internal pressure is higher than it would normally be.

The gas is preferably made to diffuse inside the beads so that the average gas pressure inside the beads is at least 0.8 × 105 Pa, for example a pressure of approximately 105 Pa.

Advantageously, the gas which is made to diffuse inside the beads comprises one or more gases chosen from nitrogen, krypton, argon and helium. For example, air can be used. The gaseous elements mentioned are chosen because of their low solubility in water and their diffusion coefficient. Of these four gases, the preferred one is nitrogen due to its low price.

Advantageously, to cause the diffusion, the pearls are placed in an atmosphere at a temperature between 200: C

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and 550 C and at a pressure of at least 5.10sPa. It is clear that the mobility of the gas molecules increases with temperature, but it will be noted that the pearls must not be subjected to a high temperature to the point that they soften and crush when they are subjected to a high pressure. It has been found that by operating in said temperature range, a favorable compromise exists between a high molecular mobility and a low risk of crushing the pearls. Operating in a high pressure environment increases the net rate of gas diffusion inside the beads. For example, hollow glass beads having a size between 10 and 250 µm and an apparent density 0.1 to 0.3 kg / 1 were treated in an autoclave maintained at 400 C and charged with nitrogen at 106 Pa. It was found that after one hour the internal gas pressure in the beads was about 105 Pa.

Preferably, diffusion occurs substantially in the absence of H2O in order to minimize the amount of water vapor that could diffuse inside the beads. This is especially important when such diffusion occurs at a temperature above the boiling point of water.

Advantageously, the treatment medium used to reduce the alkali content of the beads contains a substance chosen from HCl, HNO3, H2SO4, SO2 and SO3. The use of hydrochloric acid is particularly preferred. The use of one of these substances gives advantages by allowing a rapid dealkalization without significant risk that the well-shaped pearls are destroyed by the treatment.

Such a risk of destroying well-shaped pearls is avoided when, at the beginning of the reduction of the alkali content of the pearls, the acid treatment medium is an aqueous solution having a pH of at least 2.5, as is prefer.

In certain preferred embodiments of the invention, the hollow glass beads are subjected to a polishing treatment, for example, and preferably the beads are treated by means of a polishing solution containing an organic acid chosen from ß-hydroxy acids, oxalic acid and ethylene diamine tetracetic acid and / or a salt of such an organic acid. Such polishing removes surface defects from the glass so that the removed defects are no longer likely to create stresses. Therefore, if we compare hollow glass beads with the same wall thickness, a polished bead will have better mechanical resistance than an unpolished bead. This is important when gas is diffused inside the pearls from a high pressure atmosphere and also when the pearls are mixed in, for example, an explosive of the water-in-oil emulsion type. . In addition, because the beads are chemically polished, their walls will be thinner, which contributes to a reduction in the bulk density of the beads, which can also have significant advantages in practice. Another advantage of this chemical polishing of hollow glass beads is that a high proportion of defective beads can be eliminated. In the production of hollow glass pearls, a certain proportion of pearls are produced which are not well shaped (their walls may be incomplete or too thin in places). A chemical polishing treatment can be carried out so that too thin portions of the walls are dissolved. Therefore, defective beads can be separated by gravity in a flotation tank. Such a chemical polishing treatment constitutes the subject of British patent application 8226 106 (publication No. GB 2.127.009 A).

The β-hydroxy acids which are particularly advantageous for such a polishing treatment are, in ascending order of preference, malic acid, tartaric acid, gluconic acid and citric acid. It should be noted that, in addition to being β-hydroxy acids, these acids are also α-hydroxy acids, as is preferred. Particular preference is given to the use of a solution containing citric acid or a citrate.

When using an organic acid salt, it is preferred to use a sodium salt.

Advantageously, at the start of the treatment with organic acid and / or organic salt, the solution has a pH of at least 5 and preferably between 7 and 10. This allows good control of the speed of the reactions. and, at least when starting at the lower limit of this pH range, also promotes dealkalization of the surface of the beads.

The acid dealkalization treatment may precede or follow the treatment with the polishing solution comprising acid or the organic salt mentioned above, but it is better in practice than the acid treatment and the polishing treatment occur during overlapping periods, as preferred. The use of such an organic acid or salt in combination with an inorganic acid is particularly important since it allows significant amounts of sodium ions to be removed from the surfaces of the glass beads. The dealkalization treatment preferably begins before the polishing treatment.

In the preferred embodiments of the invention, after the acid treatment, the beads are contacted with an organosilane. Such silanization of the beads gives advantages, among others, by passivating their surfaces.

Said gas diffusion can occur before or after such a silanization of the beads.

The invention includes glass beads made by a method as described above and extends to an emulsion-type composition comprising such glass beads.

There follows a particular example of a method according to the invention.

Example

In order to produce hollow beads of vitreous material, a slip is formed, from solutions of different raw materials, which contains:

sodium silicate 100 parts by weight calcium hydroxide 2.64 parts by weight

3J sodium carbonate 3.41 parts by weight of water up to a viscosity of 2.3 Pa.sec., And urea

The slip is sprayed in the form of droplets of less than 500 µm in a combustion chamber in which they are converted into hollow beads of glassy material.

The glass beads produced have the approximately following composition by weight: Si02 70%; 25% Na20; CaO 5%. These beads are treated according to the present invention. The pearls produced have for the most part a diameter less than 125 (jm, the average diameter 45 being between 60 and 70 | im. Well-shaped pearls have a wall thickness of the order of 1 to 3 (xm, and the average pearl density is 0.35 kg / 1.

A solution is made to treat 1 m3 of these pearls by mixing 100 l of concentrated hydrochloric acid with 900 l of water and dissolving 20 kg of citric acid. The pH of the solution thus obtained is close to 0. The solution is heated to 80 ° C. in a bath and the pearls are introduced into the solution and stirred continuously for a period of 15 minutes after which the pH is approximately 7. At the At the end of this period, the pearls are removed from the treatment solution and transferred to a flotation tank containing water, where a high proportion of malformed pearls flows. The floating beads are then removed. Poorly formed pearls leak because their walls have not been properly formed or because their walls were too thin to resist the cor-rosive effect of citric acid.

At the end of the treatment, the well-shaped beads have a characteristic density of 0.28 kg / l and the alkali content of the beads, calculated as a percentage by weight of sodium oxide, has fallen from 25% to 13%.

65 It has been found that hollow glass beads made in this way contain gas, which is largely carbon dioxide, at a pressure of about one third of atmospheric pressure.

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After this dealkalization and polishing treatment, the glass beads are introduced into an autoclave which is heated to 400 ° C. to dry the beads. The atmosphere of the autoclave is cleaned to remove the water and it is connected to a nitrogen source at 106 Pa pressure. After one hour, the beads are removed and it is noted that they are charged with gas at a pressure of about 105 Pa.

Such glass beads can be incorporated, as microbubbles, in the explosive compositions described in the examples of British patent applications 1,593,163 and 2,055,358 A respectively.

As an alternative to this example, citric acid is omitted from the treatment solution. An aqueous hydrochloric acid solution with a pH of about 3.0 is made to treat beads having an apparent density of 0.3 t / m3. At the end of the treatment, the solution has a pH of around 7 and the alkali content of the pearls is reduced from 25% to 13% without substantially affecting their density.

The pearls are then treated as above.

In another variant of this example, glass beads having the approximate composition by weight: SiO2 68%; B203 7%; Na20 25%, and an actual average density of about 0.32 kg / 1 are treated in a similar manner using an aqueous solution of hydrochloric acid and citric acid.

At the end of the treatment, the well-shaped pearls have an actual average density of approximately 0.28 kg / l and the alkali content of the pearls, calculated as a percentage by weight of sodium oxide, is approximately 13%.

The beads are then treated as above.

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Claims (10)

657,355 2 1. A method of manufacturing hollow glass beads filled with gas, characterized in that it comprises a step of manufacturing hollow glass beads having an alkali content of at least 2%, calculated as a percentage by weight of oxide in pearls, a step of treating the pearls by means of an acid medium to reduce the alkali content of the pearls to a value of not more than 15%, and a step during which gas is diffused at inside the pearls. 2. Method according to claim 1, characterized in that the gas which is made to diffuse inside the pearls comprises one or more gases chosen from nitrogen, krypton, argon and helium. 3. Method according to one of claims 1 or 2, characterized in that, to cause the diffusion, the beads are placed in an atmosphere at a temperature between 200; C and 550: C and at a pressure of at least 5.105Pa. 4. Method according to one of claims 1 to 3, characterized in that the treatment medium contains a substance chosen from HCl, HNO3, H2S04, SO, and S03. 5. Method according to one of claims 1 to 4, characterized in that at the beginning of the reduction of the alkali content of the pearls, the acid treatment medium is an aqueous solution having a pH of at least 2.5. 6. Method according to one of claims 1 to 5, characterized in that the pearls are treated by means of a polishing solution containing an organic acid chosen from ß-hydroxy acids, oxalic acid and ethylene acid tetracetic diamine and / or a salt of such an organic acid. 7. Method according to claim 6, characterized in that at the beginning of the treatment with organic acid and / or organic salt, the polishing solution has a pH of at least 5 and preferably between 7 and 10 . 8. Method according to one of claims 1 to 7, characterized in that after the acid treatment the beads are brought into contact with an organosilane. 9. Glass pearls produced by a process according to one of claims 1 to B. 10. An emulsion type explosive composition comprising glass beads according to claim 9.